Measuring the long-term recovery of forest ecosystems after anthropogenic disturbances to improve restoration strategies and outcomes

Résumé

Ecological restoration is today globally implemented to mitigate forest degradation and biodiversity loss. However, restored forests do not commonly recover pre-disturbance levels of structure, functions or services. One of the obstacles for forest restoration success may be that traditional restoration guidelines are based on the short-term recovery of simple ecosystem attributes or single functions. Thus, studies of the recovery of more complex metrics over the long-term are essential to understand forest recovery process and to critically appraise forest restoration outcomes and improve their success. The main objective of this Ph. D. thesis is to assess the long-term (>100 years) recovery of forests after anthropogenic damages, by using metrics with different levels of ecological information. At a global scale, we obtained that long-term recovery of worldwide forests depends on the previous disturbance type and the recovery metric and they required more than four centuries to recover most of their biodiversity and functions. At a local scale, the species richness and diversity of ectomycorrhizal fungi interacting with beech trees and Collembola communities recovered to undisturbed values after >148 years of mine abandonment (Artikutza, northern Spain), whereas their species composition was still different. Collembola-soil fungi interaction networks from undisturbed forests presented unique species and compartmentalized and weakly connected architectures that make them more robust to extinctions than networks within the mines. Therefore, the assessment of forest restoration success should be based on there-establishment of the abiotic conditions and the diversity, composition and interactions of above- and below-ground communities to incorporate the assemblage of ecosystem complexity. Given the long time frames to recover these attributes, the current global restoration strategies should be planned following centennial timescales to set a more feasible scenario to truly restore forests.